1. Field of the Invention
The present invention relates to semiconductor cavity lasers, and more particularly, the present invention provides an improved cavity structure that uses a plurality of mirrors to separate reflection wavelengths.
2. Description of the Prior Art
Due to an arrival of an information age, using a broadband network and wavelength-division multiplexing (WDM) to provide high-capacity, high-speed data transmission has already become a trend. Traditionally, fiber-optic communication systems use wavelengths of 1.55 xcexcm and 1.3 xcexcm, because fiber loss and dispersion loss are minimized in these bands. Currently, a latest development in fiber-optic technology allows a significant reduction in hydroxyl absorption at 1.4 xcexcm. Thus, fiber-optic communications already anticipate a bandwidth at 300 nm. So, it is imperative to provide a tunable laser source.
Since beginning research of multiple quantum well structures in 1974, design of super-crystalline structure semiconductor lasers has become a separate development trend. A high efficiency, a low threshold current, and an adjustable wavelength suggest the semiconductor lasers as optimal laser light sources for use in advanced optical systems. Recently, developments in multiple quantum well laser technology already provide very wideband light sources. With an addition of an external wideband tunable technology, a single-oscillation laser can be selected, e.g. a semiconductor optical amplifier. However, tunability of the semiconductor optical amplifier is limited by the traditional resonant cavity structure.
Currently there are many tunable wavelength laser technologies, such as a Fabry-Perot (FB) wavelength filter, a diffraction grating filter, a rotated thin-film filter, an electro-optic tunable filter, and a tunable fiber-grating filter. These wavelength tuning technologies are typically used for singl-wavelength oscillation, and aside from using grating technologies, are limited to a wavelength tuning range of under 100 nm. However, if the tunable cavity semiconductor laser uses a diffraction grating technology, the tunable range increases to over 200 nm.
Please refer to FIG. 1, which is a diagram of a resonant cavity structure 100 of a prior art tunable integrated semiconductor laser. The resonant cavity structure 100 comprises a grating 10, a collimating lens 11, and an FP semiconductor laser or a semiconductor optical amplifier (SOA) 13. The FP semiconductor laser or the SOA 13 comprises a waveguide 15 for propagating light waves, a first end facet and a second end facet. The first end facet is coated with an anti-reflective layer 12, and the second end facet has a cleavage. This type of resonant cavity structure is only suitable for single-wavelength oscillation design. After the laser light is produced by the FP laser or the SOA 13, the laser light is emitted from the anti-reflective layer 12, then the collimating lens 11 focuses the laser light into one parallel beam incident on the optical grating 10. A relationship between an angle of incidence xcex8 i and an angle of reflection xcex8 r of the beam can be given as:
sin(xcex8r)=sin(xcex8i)+mxcex/xcex9
where xcex is a laser wavelength, xcex9 is a grating line spacing, and m is an integer.
When the selected laser wavelength is tuned to fit the condition xcex8 i=xcex8 r, the selected incident beam follows a path of the incident beam to reflect back to the collimating lens 11, then pass through the anti-reflective layer 12 into the FP laser or the SOA 13. This forms a single-wavelength oscillation path, and the output laser beam 14 is emitted through the second end face of the FP laser or the SOA 13.
Please refer to FIG. 2, which is a second prior art cavity structure 200 of a tunable semiconductor laser. The cavity structure 200 comprises a grating 20, a collimating lens 21 and an FP semiconductor laser or an SOA 23, which comprises a waveguide 26 for propagating light beams, a first end facet and a second end facet. The first end facet is coated with an anti-reflective layer, and the second end facet is coated with a high reflective layer 24. This type of resonant cavity structure is limited for use in single-wavelength oscillation designs. Unlike above, the laser beam is output by way of the grating 20, however the working principles are similar. After the laser light is produced by the FP laser or the SOA 23, the laser light may be reflected back by the high reflective layer 24, or directly pass through the anti-reflective layer 22. The laser light then passes the collimating lens 21, which collimates the laser light to become a parallel beamincident on the grating 20. The grating 20 thus separates the laser beam to become specific wavelength beam with an appropriate angle.
Please refer to FIG. 3. In a case of the laser beam being a dual-wavelength or multi-wavelength light source, the above two prior art tunable semiconductor laser cavity structures no longer fulfill the requirements. FIG. 3 is a diagram of a third prior art dual-wavelength resonant cavity structure 300 for a tunable semiconductor laser. The structure 300 comprises an optical grating 30, a first collimating lens 31, an SOA 32, a second collimating lens 33, a convex lens 34, a light tuning slit plate 35, and a reflector 36. The slit plate comprises a first slit 301 and a second slit 302. The optical grating 30 is set at a focal point of the convex lens 34. After the laser light source is produced by the SOA 32, the scattered light is sent through the collimating lens 31 to be focused into a parallel light beam incident on the grating 30. After being separated by the grating 30, a short-wavelength light beam 37 and a long-wavelength light beam 38 are produced, which are then sent through the convex lens 34 to become parallel light respectively incident upon the first and second slits 301,302 of the tuning slit plate 35. The beams are reflected back by the reflectors 36, thus forming two laser light resonance paths. A laser light beam 39 is sent through the second collimating lens 33 and output. Thus, the output laser beam is a dual-wavelength beam formed of the short-wavelength beam 37 and the long-wavelength beam 38. If the tuning slit plate 35 comprises further slits, by the same principle, a multi-wavelength laser beam could be produced. However, in the tunable semiconductor laser dual-wavelength cavity structure 300, it is difficult for the convex lens 34 to be accurately placed at a desired location between the optical grating 30 and the reflectors 36. If there is the slightest error in the position of the convex lens 34, not to mention the laser beam being incident on a non-ideal point on the grating 30, in either case it becomes impossible to collimate the two beams of different wavelengths. So, just using the single reflector 36 to ideally reflect the non-parallel light beams back to the resonance path is not possible. This causes non-uniform lasing loss in the dual-wavelength resonance cavity structure, and could be extended analogously to a multi-wavelength structure.
Thus, it is an objective of the claimed invention to provide a multi-wavelength tunable semiconductor laser structure that uses multiple reflectors to solve the above problems.
The structure of the claimed invention has a semiconductor optical amplifier (SOA), an optical grating, a convex lens, a light-tuning slit plate, and a plurality of tunable reflectors. The SOA has a first and a second end, the first end coupled to a resonance cavity, and the second end being a laser light output. The slit plate has a plurality of slits. The optical grating is set on a resonance path between the first end of the SOA and the slit plate. The slit plate is set in front of reflective sides of the plurality of tunable reflectors. The reflective sides of the plurality of tunable reflectors are aligned with the plurality of slits of the slit plate, respectively. The first end of the SOA through the optical grating, the convex lens, the plurality of slits of the light-tuning slit plate, to the reflective sides of the plurality of reflectors form a plurality of laser resonance paths. The plurality of tunable reflectors can independently be adjusted to different incidence angles, and are used to return non-parallel incident beams to lossless resonance paths.
It is an advantage of the claimed invention that the plurality of reflectors allows for precise tuning of multi-wavelength laser beams, reducing loss.